Any of us who travelled around the Midwest in the month of September saw a rapid change in the health of the corn crop. It seemed that this change from green to brown happened faster and earlier than in previous years, and was affecting more acres and varieties than usual. Leaf diseases such as Northern Leaf Blight, Grey Leaf Spot, and Anthracnose Leaf Blight appeared to be moving in quickly. Varieties that were average or worse in susceptibility to any of these diseases seemed to lose much of the green color in their leaves in just a few days.
So why did this happen so early and quickly this year? The answer can be explained with the disease triangle. The three components necessary for plant diseases to spread like they did this year are: 1) disease spores or inoculum in the air or soil, 2) weather conditions favorable for the diseases to spread, and 3) susceptible host plants. When all three come together, plant-killing diseases seems to spread over night. We have seen a buildup of disease spores from previous years especially in fields where growers are in a continuous corn program and growing susceptible varieties. Conditions may have not been favorable for the diseases to be of economic importance in previous years but the spores still remain and are increasing in number. This year the cool temperature at night in late August and early September resulted in heavy morning dews. This combined with cloudy, high humidity days created a perfect environment for diseases to thrive and explode.
The last component to the triangle is the susceptible host plant. Typically, a healthy corn plant can fight off many of these diseases until later in the fall when the leaves naturally start to senesce. This year, the same conditions that helped the diseases spread also had an adverse effect on the corn plants just because of the timing. August and early September are critical months for healthy corn leaves to make sugar from sunlight and convert it into starch for grain fill. This year cool, cloudy days did not allow the leaves to make enough sugar and starch in many cases. The plant had no choice but to cannibalize itself and pull sugars out of its leaves and stalks to fill its developing kernels. This stress created extreme vulnerability of the plants to the life sucking diseases that invaded the plant’s leaves and finished them off.
So what can be done to manage this situation? Crop rotation certainly helps, but that may not be an option for some. Tillage to cover diseased crop tissue can also help but again this may not be an option. Varieties do differ in their ability to resist or tolerate these diseases so varietal selection is extremely important. Nutrient balance is also very important. Potassium, for one, plays a critical role in a plant’s ability to resist disease and stress. Keep your nutrient program at an optimum so that your corn crop has a fighting chance to minimize the effects of stress and disease. Application of a systemic fungicide is also an option that should be considered following fields where significant levels of diseases were present. Hopefully, next year the environment will not be conducive for diseases to spread like we had this year. It is certain that the spores will be there. Let’s do whatever we can to minimize the susceptible host component of the triangle and control the parts of the environment that we can control.